Ground delta corner on step-up transformer?

[bentov]

At the utility service entrance point on a 208/120 system we're backfeeding a 15kva transformer to supply 480V for a submersible well pump 1,000 feet away (because the wire is too small otherwise, too difficult to change it out). Does the NEC require (or forbid) us to ground a corner of the 480V delta side?

 

[rcw retired EE]

Either method is per code, since it is not a 3 phase 4 wire system and there is no way to ground it so the voltage to ground is < 150V. Corner grounding may be the best solution, but it will increase the voltage stress on the ungrounded phases. Ungrounded may be worse.

I would be more concerned about the resulting voltage drop on motor starting with a 1000' run, the 480 V Tx impedance and the impedance of the 208/120V system. Be sure to check that. You may have to oversize the transformer and get one with taps to boost the voltage.

 

[davidbeach]

You can also get 208 delta - 480Y/277V transformers intended for just this application. Then you could ground the neutral at the transformer and just run you phase conductors. As rcwilson points out, you need to look at voltage drop, including the impedance of the transformer.